1. Mechanosensation
Domina Petric, MD

2. Functions of somatosensory system
shape and texture of objects that interact with our skin
prevention of possible injury
monitoring of the forces that are acting upon the body
proprioception (self awareness)

3. Basic organisation of somatosensory system
labeled lines (labeled line system)
mechanosensation; pain and temperature
two pairs of pathways (pair of pathways for the back of the head and body; pair of pathways for the face)

4. Peripheral mechanosensory elements
Dorsal root ganglion cells are pseudomonopolar cells.
Axon bifurcates and grows in two directions: one direction grows out to the body and other grows out to the spinal cord.
Peripheral process of pseudomonopolar cell is connected with specialised receptor ending.
The mechanosensory central process of the pseudomonopolar axon enters the dorsal root and makes a sharp upward bend and enters the dorsal column of white matter.
The pain and temperature central process of pseudomonopolar axon enters the dorsal horn and makes a synaptic connection.

5. Pseudomonopolar (pseudounipolar) neuron
Central process
Peripheral process
Peripheral process
Central process

6. Peripheral mechanosensory elements
Sensory function
Receptor type
Afferent axon type
Axon diameter (μm)
Conduction velocity (m/s)
Proprioception
Muscle spindle
Ia, II (myelinated)
13-20
80-120
Touch
Merkel, Meissner, Pacinian, Ruffini cells
Aβ (myelinated)
6-12
35-75
Pain, temperature
Free nerve endings
Aδ (myelinated)
1-5
5-30
Pain, temperature, itch
Free nerve endings (unmyelinated)
C (unmyelinated)
0,2-1,5
0,5-2

7. Mechanosensation
Meissner corpuscle and Merkel-cell neurite complex are responsible for light touch sensation.
Ruffini corpuscle is sensitive to stretch.
Pacinian corpuscles (encapsulated with many Schwann cell lamellae) are sensitive to vibration

8. Afferent systems and their properties
Small receptor field
Large receptor field
Merkel
Meissner
Pacinian
Ruffini
Location
Tip of epidermal sweat ridges
Dermal papillae (close to skin surface)
Dermis and deeper tissues
Dermis
Axon diameter
7-11 μm
6-12 μm
Conduction velocity
40-65 m/s
35-70 m/s
Sensory function
Form and texture perception
Motion detection, grip control
Perception of distant events through transmitted vibrations, tool use
Tangential force, hand shape, motion direction
Effective stimuli
Edges, points, corners, curvature
Skin motion
Vibration
Skin stretch
Receptive field area
9 mm2
22 m2
Entire finger or hand
60 mm2
Innervation densitiy
100/cm 2
150/cm 2
20/cm2
10/cm2
Spatial acuity
0,5 mm
3 mm
10+ mm
7+ mm

9. Muscle spindle, proprioception
It is formed by specialized muscle fibres and certain types of receptor endings that innervate those specialized muscle fibres.
Muscle fibres within muscle spindle are called intrafusal muscle fibres.
Intrafusal muscle fibres have collection of nuclei that are bundled near the centre of spindle shape.
Those muscle fibres have their contractile elements extending away from the central region out to pole of the muscle spindle.
Group Ia afferent axons innervate the central region of muscle spindle and group II afferent axons end in a flower spray ending on the contractile elements.

10. Muscle spindle
Wikipedia.org

11. Golgi tendon organ
Group Ib afferent axons innervate the junction of the muscle fiber and the tendon.
Golgi tendon organ is sensitive on the muscle force, while the muscle spindle is sensitive to muscle stretching.

12. Golgi tendon organ
Wikipedia.org

13. Ventral posterior complex of the thalamus
It is divided into two main nuclei:
ventral posterior lateral nucleus (somatosensory pair of pathways from posterior part of the head and body)
ventral posterior medial nucleus (somatosensory pair of pathways from face)

14. Primary somatosensory cortex in postcentral gyrus
/chudler/gif/twoptctx.gif

15. Primary somatosensory cortex (S1) somatotopy
Areas 1, 2, 3a and 3b (Broadmann´s areas).
Representation of contralateral somatosensation of foot is in the area of paracentral lobule.
Expansive representation for contralateral hand is in the S-shape area of the central sulcus.
The face is conveyed by inputs from the ventral posterior medial nucleus of the thalamus which terminates in the inferior segment of the postcentral gyrus, below the S-shape bend of the central sulcus.

16. Thalamic Ventral Posterior Medial Face Nucleus Thalamic Ventral
Lateral
Nucleus
Paracentral lobule
S-shape bend of the central sulcus
Thalamic
Ventral
Posterior
Medial
Nucleus
Face
(Neuroscience 5th edition. Sinauer Associates, Inc )

17. Cortical magnification
How much more does cortical circuits magnify the representation of the body that is related to the density of peripheral receptors and the density of neurons at all the antecendent stations in the somatic sensory pathway.

18. Primary somatosensory cortex (S1) somatotopy
Area 3a is concerned with proprioceptors (muscle spindle, golgi tendon organ, joint receptors) and proprioception.
Area 3b is concerned with cutaneous receptors.
Area 1 is going to respond to complex stimuli activating multiple skin surfaces.
Area 2 is concerned with the shapes of objects that we encounter.

19. Secondary somatosensory cortex
Secondary somatosensory cortex is in the inferior and posterior aspect of the parietal lobe and recieves inputs from primary somatosensory cortex.
Informations are then passed to amygdala and hippocampus (creation of memories).
All areas project to secondary somatosensory cortex, but area 2 also projects to parietal areas 5 and 7 (orientation relative to the environment).

20. Secondary somatosensory cortex (S2)
Amygdala, hippocampus
Secondary somatosensory cortex (S2)
Parietal areas 5 and 7: orientation
Area 3a: proprioception
Area 3b: cutaneous receptors
Area 1: complex stimuli
Area 2: shapes of objects

21. Mechanosensory pathways
II.

22. Two pairs of somatosensory pathways
Post-cranial body: posterior part of the head and body below the head
Dorsal column medial lemniscal system: mechanical stimuli
Anterolateral system: pain and temperature
Face and anterior portion of the head
Pathway through the principal sensory trigeminal complex nucleus: mechanical stimuli
Pathways through the spinal trigeminal complex nucleus: pain and temperature

23. Dorsal column medial lemniscus system, mechanosensation from the post-cranial body
First order neuron is dorsal root ganglion neuron in the spinal cord.
Central process enters the dorsal column in the white matter and runs the longitudinal length of the spinal cord.
Dorsal column nuclei in the dorsal part of tegmentum of medulla oblongata: synapses for the second order neuron.
Axons of the second order neuron cross the midline.
Third order neuron is in the ventral posterior lateral nucleus of the thalamus.

24. First order neuron: dorsal root ganglion neuron
Third order neurons: ventral posterior lateral nucleus of the thalamus
After crossing the midline, fibres continue as medial lemniscus.
Second order neuron: dorsal column nuclei in the tegmentum of medulla
Crossing the midline
Internal arcuate fibers are crossing
the midline.
Dorsal column
of the spinal cord
Fibers from cuneate tract (upper
extremity) project to the middle
of the postcentral gyrus.
Postcentral gyrus
Second order neurons
are in cuneate nucleus
and gracile nucleus.
First order neuron: dorsal root ganglion neuron
Fibers from gracile tract
(lower extremity) project to the
paracentral lobule.
Dorsal column medial lemniscus system: mechanosensation from the post-cranial body.

25. Trigeminal lemniscus pathway or V-lemniscus, mechanosensation from the face
First order neuron is the ganglion cell in the trigeminal ganglion.
Axons enter the brainstem through the trigeminal nerve.
Second order neuron is in the principal sensory trigeminal complex nucleus.
Second order axon crosses the midline and ascends the remaining divisions of the brain stem, the upper pons and the midbrain and enters the ventral posterior complex of the thalamic nuclei.
This pathway is trigeminal lemniscus and it is near the medial edge of the medial lemniscus.
Third order neuron is in the ventral posterior medial nucleus.
Third order neuron projects to the inferior one third of the postcentral gyrus.

26. Inferior one third of postcentral gyrus
Third order neuron: ventral posterior medial thalamic nucleus.
Second order neuron: principal sensory trigeminal complex nucleus
Axons continue ascending as trigeminal lemniscus.
Axons cross the midline.
First order neuron: trigeminal ganglion
Inferior one third of postcentral gyrus
Trigeminal lemniscus pathway: mechanosensation from face and anterior portion of the head.

27. Cerebellum
It helps to facilitate agile movements of the body and agile movements of thoughts.
The cerebellum gets inputs from the cortex about what we are trying to do.
The cerebellum gets feedback from our sensory systems about what we are actually doing.
If there is a problem, the cerebellum will generate an error signal that is sent back into our motor system for an adjustment (correction can be made).

28. Spinocerebellar pathways: dorsal spinocerebellar tract
Receptor is for example proprioceptor in skin of the foot.
The first order neuron is in the dorsal root ganglion of the spinal cord.
First order axon enters the dorsal column and ascends to the thoracic spinal cord (from lower extremity).
Clarke´s nucleus is in the intermediate gray matter of the thoracic spinal cord: synapse occurs with first order axon.
Clarke´s nucleus extends throughout the thoracic spinal cord and then to the upper lumbar levels of the spinal cord.

29. Spinocerebellar pathways: dorsal spinocerebellar tract
Clarke´s nucleus gives rise to a second order axon that ascends in the dorsal lateral white matter of the spinal cord.
This is dorsal spinocerebellar tract.
The dorsal spinocerebellar tract ascends to the cerebellum via the inferior cerebellar peduncle.
This is the pathway for lower extremity.

30. Spinocerebellar pathway for upper extremity
From the upper extremity, first order axons ascend in the dorsal column.
From the dorsal column fibres interact with external cuneate nucleus in medulla oblongata
From external cuneate nucleus second order axons project to the cerebellum.

31. Conscience awareness
Proprioceptive informations from spinocerebellar tracts enter the medial lemniscus via colateralls and enter the conscience awareness.

32. Literature
Leonard E. White, PhD, Duke University
Neuroscience 5th edition. Sinauer Associates, Inc
Wikipedia.org